This section is intended to introduce the reader to various aspects of art, which may be related to various aspects of the present invention that are described below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present invention. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
Today, most customer homes receive a number of programs containing video and audio content, as well as a large amount of data, from a plurality of sources, such as broadcast television, cable, satellite, digital subscriber line systems. These systems often utilize distribution networks to deliver the programs and content to the customer premises. Many distribution networks carry content on multiple channels or transponders that may originate from different sources (e.g. multiple satellites, cable head-ends, etc). The multiple channels or transponders from different sources are brought together on a single medium (e.g. coaxial cable) before going, for example, into a customer's home for connection to a television or settop box. As a result, a frequency map identifying the incoming channels or transponders, as well as certain characteristics, such as data symbol rates or signal bandwidths, of each channel or transponder may not be completely known initially.
In instances where the frequency map is not completely known, the television or settop box may be designed to perform some form of channel or transponder search in order to identify channels or transponders, if possible along with channel or transponder characteristics, so that the television or settop box can proceed to properly demodulate and decode the signals. In one method the television or settop box may implement a blind transponder or channel search. In a blind search the signal is filtered by varying-bandwidth filters and then the demodulator in the link circuit attempts to demodulate the filtered signal. The demodulation is performed under assumption that the filtering was successful in presenting a single transponder or channel of interest at the demodulator input while sufficiently suppressing adjacent ones. Further the demodulation will be successful only if the frequency offset and symbol rate offset of that single transponder or channel are within pull-in range of the corresponding phase locked loops (PLLs) in the demodulator. The blind search continues through all possible combinations until all channels or transponders in the incoming signal have been searched. The blind search may create accurate identification results but is very slow and time consuming to apply.
In another approach, a Fast Fourier Transform (FFT) processor is used to perform either a complete or a piece-wise spectral analysis of the incoming signal. A subsequent analysis of the spectrum may include identifying signals based on characteristics such as the root-raised-cosine response or some other spectral shaping of each transponder or channel. The spectral characterization may allow identification of a transponder or channel along with its location and an estimate of its symbol rate or bandwidth. However, the uneven power distribution among the transponders or channels and the presence of high levels of additive noise, as well as the ubiquitous multi-path conditions that distort the signal spectrum may hinder the direct use of an FFT. As a result, the FFT approach ultimately may yield similar results for accuracy and speed compared to the above blind method.
The problem with the existing search approaches is further exacerbated by a system containing a large number of transponders or channels from multiple sources with many possible signal types. The above mentioned approaches can take an unacceptably long period of time to perform the search, detection, and identification. Further, if the network requires frequent re-initialization due to changes in the frequency map, the period of waiting for channel or transponder detection may be unacceptable to the user. Therefore, there is a need to perform improved transponder or channel detection, determination, and identification in a signal.